The incorporation of sulfur into a hydroxybenzoate of an alkaline earth metal was first attempted by Orland M. Reiff, as disclosed in U.S. Pat. No. 2,256,443 (1941), in which an alkali metal alkylsalicylate obtained by the Kolbe-Schmitt process was reacted with sulfur chloride in the presence of butyl alcohol solvent to incorporate sulfur into the salicylate, and then the resulting salicylate was converted into an alkaline earth metal salt using an alkaline earth metal alcoholate. This method was distinguished for the suppression of hydrogen chloride generation.
For incorporating sulfur, Jerome M. Cohen, as disclosed in U.S. Pat. No. 3,595,791 (1971), used a method comprising metathetically reacting an alkali metal alkylsalicylate obtained by the Kolbe-Schmitt process with an alkaline earth metal halide to convert the alkali metal salt into an alkaline earth metal salt and reacting it with elemental sulfur in the presence of a Carbitol represented by the formula R(OR.sub.1).sub.x OH and of an alkaline earth metal oxide or hydroxide or a mixture of both (hereinafter referred to as "alkaline earth metal reagent"). This method is distinguished for the use of elemental sulfur, which is easily handled, in place of an extremely highly reactive sulfurizing reagent such as sulfur chloride.
However, the methods proposed by Reiff and Cohen each has had the following drawbacks to the industrial use thereof. The first point is that each process is complicated with a large number of steps. The Reiff process necessitates reconversion of the product of the sulfurization reaction into a free acid, while the Cohen process necessitates metathesis with an alkaline earth metal halide after the Kolbe-Schmitt reaction. Thus, such steps make the processes more complicated. The second point is that each process involves a step in which an alkali metal halide is generated as a by-product; inclusion of such a strong electrolyte into the product is undesirable from a quality standpoint.
On the other hand, a reaction in which an alkaline earth metal complex of an alkylphenol and carbon dioxide are used in combination has been utilized in the field of the phenate industry which is competing with the salicylate industry (see Nishikawa and Ishibe, PETROTECH, 7, 338 (1984)).
It has been generally thought that the reaction involving such a combination does not yield a salicylic acid compound, as already reported by John S. Bradley et al., as disclosed in U.S. Pat. No. 2,916,454 (1959). Accordingly, the present inventors succeeded in obtaining a mixture of sulfurized hydroxyalkylbenzoate of an alkaline earth metal and an alkylphenol and obtaining a process for producing the mixture, by mixing and reacting an alkaline earth metal oxide with an alkylphenol and a dihydric alcohol, subsequently distilling off water and the dihydric alcohol, treating the thus-obtained alkaline earth metal phenate with carbon dioxide in the presence of an alkylphenol to form a hydroxyalkylbenzoate, and then reacting it with elemental sulfur, as disclosed in U.S. Pat. No. 4,902,436 (1990). This method, which explodes the established theory that the presence of a phenol prevents the formation of an alkylsalicylic acid (e.g., James Hartley, British Patent 734,622 (1955), page 1, line 34 et seq.), is distinguished for the significantly simplified process which is attained by yielding an alkaline earth metal hydroxyalkylbenzoate directly from an alkaline earth metal phenate without the necessity of using an alkali metal.
The above method, however, has had drawbacks that the total base number of the product obtained is relatively low from an operation efficiency standpoint and decarboxylation reaction is apt to take place in the sulfurization step, and that the product obtained has a considerably dark color.
Furthermore, in the process according to the present invention, when the sulfurization reaction is conducted at atmospheric pressure or in a pressurized closed system, it has been disadvantageous in that the recovered phenols after the reaction become milky. It is preferred to be capable of reusing the recovered phenols. If they cannot be reused, a further considerable cost becomes necessary since the process requires a great amount of phenols. Though the reason why the recovered phenols become milky is uncertain, it is supposed that in producing a mixture of sulfurized alkaline earth metal salts of a salicylic acid and a phenol, the sulfurization is conducted in the presence of a dihydric alcohol in the latter reaction stage, whereby by-products such as polysulfides contaminate the recovered phenols. The resulting milky phenols are low in a commercial value as phenols. Moreover, it is expected that the reuse of the milky phenols would cause undesirable side reactions in the production of a phenate and a salicylate, and the milky phenols cause a reduction in the oil solubility of a final product.